1. Field of the Invention
The present invention relates to a method and a device for manufacturing a semiconductor thin film especially with controlled grain boundaries and to a thin film transistor.
2. Description of the Related Art
As a switching device for constituting pixels in a liquid crystal display device, used is a thin film transistor (referred to as a “TFT” hereinafter) formed on a glass substrate. Recently, in addition to achieving highly fine liquid crystal display devices, there has been an increasing demand for improving the action speed of the TFTs in order to achieve a system-on-glass, and a technique for forming a high-quality laser annealed polycrystalline silicon TFT has drawn an attention.
The above-described TFT is manufactured in the manner as shown in FIG. 1. For example, as shown in FIG. 1(1), amorphous silicon 1201 is formed on an insulating film 1202 which is formed on a surface of a glass substrate 1203. Then, as shown in FIG. 1(2), a polycrystalline silicon 1201′ is formed by irradiating a laser light 1204 onto the surface of the amorphous silicon 1201. Subsequently, as shown in FIG. 1(3), a source region 1207, a drain region 1209, and a channel (active layer) 1208 sandwiched in between the source region 1207 and the drain region 1209 are formed on the obtained polycrystalline silicon 1201′. A gate insulating film 1212 and a gate electrode 1206 are formed thereon. After forming an interlayer insulating film 1211 by covering the gate electrode 1206 and the gate insulating film 1212, a contact hole going through the interlayer insulating film 1211 and the gate insulating film 1212 is formed. Then, on the interlayer insulating film 1211, a source electrode 1205 connected to the contact hole of the source region 1207 and a drain electrode 1210 connected to the contact hole of the drain region 1209 are formed, respectively. Thereby, the TFT is completed.
Recently, there has been a still increasing demand for a further improvement in the action speed of the polycrystalline TFT. The action becomes faster when the mobility of carrier (electron or hole) within a channel becomes larger. However, when there are a large number of grain boundaries present within the channel, the mobility of the carrier is decreased. Therefore, techniques for improving the mobility of the carrier have been proposed as described below, in which the number of grain boundaries within the channel is decreased by controlling crystal growth at the time of laser annealing.
First Related Art
Disclosed in “Sequential lateral solidification of thin silicon films on SiO2” (Robert S. Sposili and James S. Im, Appl. Phys. Lett 69 (19) 1996 pp. 2864-2866) is a technique for forming huge crystal grains in a direction of scanning by scanning a narrow-line beam. This technique will be described in the followings.
First, as shown in FIG. 2(1), pulse laser light is shaped into a narrow-line beam 1302 by a prescribed mask, and the shaped narrow-line beam 1302 is scanned along a substrate to be irradiated onto amorphous silicon 1301 of the substrate. Thereby, the amorphous silicon 1301 is heated (annealed) in order.
As shown in FIG. 2(2), by the first irradiation of the narrow-line beam 1302, crystallization of the dissolved amorphous silicon film proceeds as follows. First, each crystal grows towards the center of the dissolved region with the end portion of the narrow-line scanning direction (the beam width direction), which is an interface of solid and liquid phases between with the adjacent undissolved region, being the start point. As a result, the solidified portion becomes the crystallized polycrystalline silicon 1301′. Further, each crystal collides in the center area and the vicinity and the growth is interrupted, thereby forming the crystal grain boundaries in these areas. In the direction (beam length direction) vertical to the scanning direction, a large number of crystal grain boundaries are generated along with the scanning direction.
Subsequently, as shown in FIG. 2(3), performed is the second irradiation of the narrow-line beam 1302′. The scanning amount of the second narrow-line beam 1302′ is equal or smaller than the grain size of the crystal grain crystallized along the scanning direction of the first narrow-line beam 1302.
Then, as shown in FIG. 2(4), in accordance with the irradiation of the second narrow-line beam 1302′, crystal growth is carried out using the crystal grains grown by the first irradiation as a seed.
By repeating the dissolving and crystallization of the amorphous silicon 1301 by scanning the laser irradiation region in order, a crystal grain 1303 extending along the scanning direction can be formed as shown in FIG. 2(5).
Second Related Art
Japanese Patent Unexamined Publication No. 11-64883 discloses a technique for scanning and irradiating by shaping a light beam into a zigzag beam shape by letting the beam pass through a transmission section 1401 using a shielding mask which comprises a shielding section 1402 and the transmission section 1401 in a zigzag pattern shown in FIG. 3(1). In this technique, not only the growth in the scanning direction but also the crystal growth in the direction vertical to the scanning direction is performed with the peak of the beam pattern being the start point. As a result, as shown in FIG. 3(2), it has been reported that it is possible to form a crystal grain 1502 in which the positioning is controlled in accordance with the cycle of the zigzag pattern. In FIG. 3(2), a reference numeral 1501 is a high-density grain boundary region and 1503 is a crystal grain boundary.
In the case of laser annealing as described in the first related art, it is possible to extend the crystal grains in the scanning direction of the laser light (in the beam width direction). However, there is no temperature gradient in the direction (the beam length direction) orthogonal to the scanning direction of the laser light so that the crystal grain boundaries are generated at random in the beam length direction. Therefore, there may cause such shortcomings that the growth of the crystal grains are interrupted and that the grain+in the beam length direction becomes as short as 1 μm. As a result, when TFTs are manufactured by providing channels so that the carriers move in parallel to the scanning direction, there are crystal grain boundaries generated in the channels since the positions of the crystal grain boundaries are not controlled. Thus, the mobility of carrier is deteriorated and fluctuations in the mobility and threshold voltage are increased. Further, when the TFTs are manufactured by providing the channels so that the carriers move in the direction vertical to the scanning direction, there are crystal grain boundaries generated in the channels by interrupting the transition of the carriers since the positions of the crystal grain boundaries are not controlled. Thus, the mobility of carrier is deteriorated and fluctuations in the mobility and threshold voltage are increased.
Further, protrusions are generated along the crystal grain boundaries in each scanning step. Since the crystal grain boundaries in the beam width direction are formed at random, positioning of the protrusions in the beam width direction becomes random. In the TFT including the protrusions within the channel, the electric fields at the time of action are concentrated in the protrusions, thereby causing the fluctuation of the threshold voltage. That is, the dispersion in the threshold voltage becomes large in the TFT manufactured in the first related art, in which the positioning and the number of the protrusions within the channel become random.
In the laser annealing using a shielding mask as described in the second related art, the beam shape on the shielding mask in general is in a rectangular shape (a laser irradiation region 1403) as shown in FIG. 3(1). Thus, when the laser is let through the mask in the zigzag pattern, the transmittivity of the laser light is decreased compared to the case of the first related art where the narrow-line beam is formed. As a result, the beam length irradiated onto the substrate becomes shorter and the polycrystalline region attained in one-time scanning irradiation becomes narrower. Therefore, the time required for processing the substrate is extended.
Further, in the obtained crystal, a high-density grain boundary region 1501 as shown in FIG. 3(2) is generated in a wide range in the start position and the end position of the irradiation. Also, in a step of manufacturing the mask, forming a complicated zigzag pattern increases the cost compared to the case of forming a straight-line pattern. In addition, an optical system with high resolution becomes essential to a laser annealing device in order to form the zigzag pattern beam.